1. Field of the Invention
The present invention relates to a suspension apparatus for an automotive vehicle.
2. Description of the Related Art
A member for supporting a steering wheel, that is, a knuckle, is supported to the body of the vehicle at least through a lower arm so as to be pivotable in vertical directions. The wheel to be mounted to the member for supporting the wheel is required to be aligned in a predetermined way, and one of the significant factors for determining the alignment of the wheel is a kingpin axis.
The kingpin axis is arranged in such a manner that, when the body of the vehicle is viewed from its front side or its rear side, an intersection of the downward extension of the virtual kingpin axis with the road surface with which the wheel is in contact exists the position to some extent outside an intersection of the transversely central line of the wheel with the road surface. It is further preferred that the wheel be toed in. In addition, it is desired that an intersection of the virtual kingpin axis with a horizontal extension from the rotational center of the wheel be located in the position to some extent inside the body from the rotational center of the transversely central line of the wheel and the wheel is toed in when driving force is applied. Such settings are strongly required particularly for a front suspension of a front wheel drive car having an engine capable of generating a large degree of torque.
As is understood from the foregoing description, it is required that the kingpin axis is set so as to be inclined to some extent toward the outside of the body as it extends downward, when the body of the vehicle is viewed from its front side. For instance, for a strut type suspension, the portion of connection between the lower arm and the member for supporting the wheel constitutes a lower set point and the portion of connection between a suspension damper and the member for supporting the wheel constitutes an upper set point. Further, the line connecting the lower set point to the upper set point is set as a kingpin axis.
In order to satisfy the requirements for inclination of the kingpin axis in the manner as described hereinabove, it is necessary to extend the lower set point, i.e. the transversely outer end portion of the lower arm, into the long depth of the inner radial portion of the wheel, that is, towards the transversely outer side. It is difficult, however, to extend the lower arm to a great extent toward the outside in the transversely circumferential direction from the point of view of prevention of the interference with brake disks, and so on.
A suspension of a so-called double pivot type is proposed in Japanese Patent Examined Publication No. 5842,042 or its U.S. counterpart, i.e. U.S. Pat. No. 4,145,072, in which the lower arm is divided into a front lower arm and a rear lower arm to thereby provide two connections of the member for supporting the wheel to the front lower arm and to the rear lower arm, in order to locate the lower set point of the kingpin axis in the position toward the transversely outer side of the body. In other words, the proposed suspension is structured in such a manner that the lower set point of the virtual kingpin axis is constituted by an intersection formed by the axial extension of the front lower arm and the rear lower arm toward the transversely outer side of the body. More specifically, a phantom line connecting the connecting portion of the front lower arm at the side of the body to the connecting portion thereof at the side of the wheel-supporting member intersects with a phantom line connecting the connecting portion of the rear lower arm at the side of the body to the connecting portion thereof at the side of the wheel-supporting member, and the point of the intersection is substantially set as the lower set point of the virtual kingpin axis by locating the resulting intersection between the two phantom lines in the position at the side of the transversely outer end portion from the connecting portion between each lower arm and the member for supporting the wheel.
When the virtual kingpin axis is constituted by the division of the lower arm into the front lower arm and the rear lower arm, however, this system may present the problem that the ability of returning the steering wheel becomes poor, when the wheel is steered, because of the small castor trail of the externally cornering wheel, i.e. the wheel located at the outer side of cornering.
More specifically, as the steering wheel is steered, the externally cornering wheel is steered in a moment about the virtual kingpin axis that was set when the wheel was in a straight forward state. At this time of steering, the connecting portion of each of the front lower arm and the rear lower arm to the member for supporting the wheel is displaced relatively rearward as compared with the straight forward state of the wheel, thereby causing each of the lower arms to pivot toward the rear side of the body of the vehicle and displacing the lower set point of the virtual kingpin axis in the rearward direction. As a result, the castor trail of the externally cornering wheel becomes smaller so that the ability of returning the steering wheel becomes poor. In particular, as the externally cornering wheel functions as the wheel for substantially governing the return of the steering wheel by migration of the load of the body of the vehicle, the returnability of the steering wheel becomes considerably poor during driving at a high speed and a large steered angle, such as during sport running In extreme cases, the situation may arise where the wheels are steered to an extent larger than intended.